Definition of "nightdress"

The window of the room was open, and beside the window, all huddled up, lay the body of a man in his nightdress.

1887, A[rthur] Conan Doyle, “A Study in Scarlet”, in Beeton’s Christmas Annual, London, New York, N.Y., Melbourne, Vic.: Ward, Lock & Co., part I (Being a reprint from the reminiscences of John H. Watson, M.D., […]), page 44